1. Field of the Invention
The present invention relates generally to a frequency offset compensation apparatus for an OFDM/CDMA (Orthogonal Frequency Division Multiplexing/Code Division Multiple Access) system, and in particular, to a frequency offset compensation apparatus which compensates for a frequency offset (or frequency error) using a guard interval and a pilot symbol.
2. Description of the Related Art
As the types of the recent multimedia services are diversified, it is necessary to transmit data at high speed. In addition, as the user's demand for construction of a wireless network increases, a wireless asynchronous transmission mode (hereinafter, referred to as “WATM”) market is expanded. Thus, every country forms various organizations for WATM standardization to expedite implementation of the WATM technology. For implementation of such a high-speed data transmission technology, active researches are being carried out on a method for using the orthogonal frequency division multiplexing (hereinafter, referred to as “OFDM”) technology in implementing the high-speed data transmission. In the OFDM technology, data is transmitted on a plurality of subcarriers after inverse fast Fourier transform (IFFT), and the transmitted subcarriers are converted to the original data in an OFDM receiver through fast Fourier transform (FFT).
FIG. 1 illustrates a structure of a general OFDM/CDMA system. With reference to FIG. 1, a description will be made of the structure and operation of a transceiver in the OFDM/CDMA system.
First, the structure of a transmitter will be described. A spreader 101 spreads data symbol streams to be transmitted by multiplying the data symbol streams by a code of an N rate in a data symbol unit. Herein, N data bits obtained by multiplying the data symbol by the code of N rate will be referred to as “data samples”. The N data samples spread from the data symbol are parallelized by a serial-to-parallel (S/P) converter 103 and then, input to a pilot sample inserter 105. The pilot sample inserter 105 receives the N data samples in parallel, punctures the received data samples at regular intervals, and then inserts pilot data samples as shown in FIG. 2, and the pilot sample-inserted data symbol is provided to an inverse fast Fourier transform (IFFT) section 107. The IFFT 107 receives in parallel the pilot sample-inserted data samples in the data symbol unit and performs inverse fast Fourier transform on the received data samples. In the following description, the IFFT-transformed data output from the IFFT 107 will be referred to as “OFDM symbol”. The OFDM symbol is also comprised of N data samples. The OFDM symbol output from the IFFT 107 is input to a guard interval inserter 109. The guard interval inserter 109 copies a part of the rear end of the received OFDM symbol and inserts it in the front of the OFDM symbol. The guard interval-inserted OFDM symbol is converted to an analog OFDM symbol by a digital-to-analog converter (DAC) 111 and the converted analog OFDM symbol is transmitted after up-conversion.
Next, a receiver down-converts the analog signal transmitted from the transmitter. Because of the inaccuracy of an oscillator used during the down-conversion, the baseband signal includes a frequency offset. The analog signal is converted to a digital OFDM symbol by an analog-to-digital converter (ADC) 121 and then, applied to a guard interval remover 123. The guard interval remover 123 frame-synchronizes the OFDM symbol output from the ADC 121, and after frame synchronization, removes the guard interval included in the OFDM symbol, the guard interval-removed OFDM symbol being applied to a fast Fourier transform (FFT) section 125. The FFT 125 FFT-transforms the OFDM symbol output from the guard interval remover 123 and outputs a data symbol. At this point, since a signal is obtained which is shifted by the frequency offset included during the down-conversion, it is difficult to recover the original data. Particularly, in an OFDM/CDMA system where a desired signal is carried at each frequency band, the frequency offset should be correctly estimated and compensated for to recover the original signal. To compensate for the frequency offset, a carrier synchronizer 127 detects a pilot sample from the data symbol output from the FFT 125, and performs carrier synchronization using the detected pilot sample. A despreader 129 despreads the data symbol output from the FFT 125, which was spread into N data samples, and outputs the original data symbol.
The FFT 125 generally recovers the frequency offset using the FFT characteristics shown in Equation (1) below.
                                          X            ⁡                          [              n              ]                                ⁢                      W            N                                          K                0                            ⁢              n                                      ↔                              X            ⁡                          [                              k                -                                  k                  0                                            ]                                ⁢                      (                                          W                N                            =                              ⅇ                                                                            -                      j2                                        ⁢                                                                                  ⁢                    π                                    N                                                      )                                              (        1        )            where X[n] is an input signal in a time domain, which is input to the FFT, WNK0n is an offset term, and X[k-k0] denotes a received signal with a frequency offset, which is shifted by k0 from the transmission signal during down-conversion.
FIG. 2 illustrates a data structure used in the general OFDM/CDMA system, which shows that the pilot data samples are inserted after puncturing N data samples for each data symbol in a specific pattern. Since the pilot data samples are inserted in a specific pattern, Equation (1) is calculated using the pilot data samples and the frequency offset is compensated for by calculating a shift amount k0 of the data calculated by Equation (1).
In an ideal system, since the pilot samples received as shown in Equation (1) are received in a position shifted by k0 samples from the original reference sample position, it is possible to calculate the frequency offset k0 by estimating the shifted value using a correlator. However, in the OFDM/CDMA system, use of the above pilot samples causes such performance degradation as an increase of over 2 times in a data rate, complication of a receiving stage for compensating for the frequency offset, and an increase in a noise level, so that it is difficult to use the pilot samples.
A non-ideal system has the more serious problems. The factors affecting the IFFT-transformed signal include a timing error, a common phase error (CPE) and the noises. In the receiver, a timing error ne in a time domain, after passing the FFT stage, are expressed by the product of the original signal in the frequency domain and an exponential term. This ultimately affects even the pilot sample value, so that an increase of this value may cause considerable performance degradation of the correlator. Therefore, in the OFDM/CDMA system, it is difficult for the conventional frequency offset compensation method to detect a correct frequency offset value.